In recent years, for producing various devices, thin film formation technologies have been widely employed. For example, for solar cells and lithium ion batteries, there has been attempted to use silicon thin films. Such use of thin films is important not only in view of improvement in functionality and reduction in size of devices but also in view of conservation of global environment, for example, conservation of resources, reduction in power consumption, and the like. Under these circumstances, the improvement in productivity and stability of thin film formation and the reduction in costs have been required.
In improving the productivity of thin films, a technology that enables continuous film formation for a long duration of time is indispensable, and for vacuum vapor deposition, a continuous supply of a material for forming a thin film to an evaporation source has been required. However, the continuous supply of a material for forming a thin film involves possibilities that the temperature of the evaporation source and the evaporation rate may fluctuate, adversely affecting the stability of the film formation.
As a solution for suppressing the fluctuation in temperature of the evaporation source that occurs in association with the continuous supply of a material for forming a thin film, a method of melting a material for forming a thin film beforehand and supplying the material in a droplet state to the evaporation source has been known. In such a supplying method, for allowing the material to melt quickly, it is beneficial to use a material having a small cross section and having a large ratio of the length to the diameter or width. Specifically, the material for forming a thin film preferably is a rod-shaped member having a width (or a diameter) of about 30 to 100 mm, and a ratio (H/D) of length H to width (or diameter) D of about 10 to 30.
As a method of producing a material for forming a thin film, such as silicon, into a rod-shaped member, a casting method is effective.
However, for example, silicon expands when solidified since the density of solid phase thereof at room temperature is small as compared to the density of liquid phase thereof at a temperature close to the solidification point. This results in various problems such as impossibility of removal of the cast silicon from the mold and breakage of the mold due to the stress during solidification. Even if the breakage of the mold is avoided, there is another problem in which the cast rod of silicon is broken inside the mold, since the cast article is a rod-shaped member (a cast rod) whose strength in the direction orthogonal to the longitudinal direction (i.e., in the diameter direction) is not strong.
Patent Document 1 suggests an integrated mold for casting polycrystal silicon ingot, the mold being provided with a predetermined upward enlarging taper in its sidewall. Patent Documents 2, 3 and 4 disclose assembled molds for casting silicon.
Patent Document 5 discloses a mold for continuous casting including a movable die comprising shiftable elements and fixed elements secured to a frame, wherein the boundary end surfaces of the shiftable elements are formed in parallel with the shifting direction thereof.
Patent Document 6 discloses an adjustable mold whose inner diameter is adjustable by hydraulic cylinders located in the radius direction of the casting cross section.
However, the integrated mold as disclosed in Patent Document 1 is characterized by producing an ingot having an increased base area and a reduced height, and therefore is not suitable for producing a cast rod. Further, in the case of the integrated mold, in general, it is necessary to destroy the mold to remove the cast article therefrom. This makes the recycling of the mold impossible, making it difficult to reduce the production costs of cast articles.
On the other hand, in the case of the assembled molds as disclosed in Patent Documents 2 to 4, it is not necessary to destroy the mold, to remove the cast article therefrom. However, the mold repeatedly undergoes stress associated with expansion during solidification, and thus the member for securing the mold and the like are easy to break, and the repeated use of the mold is difficult. Moreover, none of the assembled molds as disclosed in Patent Documents 2 to 4 are designed for producing a cast rod. If a cast rod is produced with the use of a material that expands when solidified, such as silicon, the cast rod may be broken inside the mold, reducing the yield.
Both of the molds as disclosed in Patent Documents 5 and 6 are designed for continuous casting. In the case of these molds for continuous casting, in order to allow the dies, through which a molten material passes continuously while cooled, to be held in contact with the cast piece that will contract as cooled, the mold is merely provided with a mechanism for changing the diameter of the mold. Such a mold for continuous casting is not suitable for producing a cast rod having a large length as compared with the width (diameter).
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 10-190025
Patent Document 2: Japanese Laid-Open Patent Publication No. Sho 62-108515
Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 10-182285
Patent Document 4: International Publication No. 2005/073129 Brochure
Patent Document 5: Japanese Laid-Open Patent Publication No. Hei 1-218741
Patent Document 6: Japanese Laid-Open Patent Publication No. Hei 4-200845